Since it originally emerged in late 2019, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is the virus responsible for the coronavirus disease 2019 (COVID-19) pandemic, has spread across the world and infected over 251 million people to date. The course and symptoms of the disease can range from mild and asymptomatic to severe clinical progression.
Study: Long‑lasting olfactory dysfunction in COVID‑19 patients. Image Credit: Yuganov Konstantin / Shutterstock.com
Olfactory dysfunction (OD) and gustatory dysfunction are common symptoms seen in COVID-19 patients. Several studies have reported chemosensory dysfunction as primary OD in COVID-19 patients.
The prevalence of OD in COVID-19 patients varies due to differences in the evaluation methods and psychophysical tests in different geographic regions. Many patients have been found to regain olfactory function within the first month; however, long-term results vary.
Long-term prevalence of OD in COVID-19 patients
A recent study published in the European Archives of Oto-Rhino-Laryngology discusses the long-term prevalence of OD in COVID-19 patients. The authors studied the course of chemosensory function in COVID-19 patients within 3 - 15 months after they were diagnosed with COVID-19.
The single-centered study enrolled a total of 102 patients diagnosed with laboratory-confirmed COVID-19 with an average age of 38.8 years. Once patients performed chemosensory tests at home, further psychophysical tests were performed at the clinic.
Questionnaires on the importance of olfaction (IOQ) and olfactory-specific quality of life (QOD) were given at the beginning and the end of the study. The authors observed that the mean Snifn’ Sticks (TDI) score was 27.1 points, among which 4.0% were anosmic, 72.5% hyposmic, and 23.5% normosmic.
The follow-up testing showed that 73.5% of patients had improvement in OD, 5.9% had deterioration, and 20.6% had no change in OD. However, full recovery of self-perceived smell, flavor, and taste was not observed.
The study reported three significant findings. First, assessing the olfactory function revealed that the majority of the patients had hyposmia up to 15 months after the onset of OD. Second, improved self-assessed smell function was observed when compared to the initial olfactory loss but not to the level of olfactory function before the disease.
Moreover, full recovery of self-perceived smell, flavor, and taste was not observed. The questionnaire responses showed that the importance of smell did not change at the individual level. However, there was an improvement in OD-related quality of life and the parosmia scores increased at follow-up. Also, the severity of OD decreased significantly with time.
The study provided further evidence to show that a high proportion of COVID-19 patients suffering from loss of smell do not recover rapidly, which contradicts earlier reports on OD in COVID-19 patients. The authors believe that even though some COVID-19 patients suffer from OD for a long time, the olfactory epithelium is not disrupted permanently, and a subgroup of olfactory neurons remains still intact. The study also reported only a weak to moderate correlation between psychophysical and self-assessed evaluations of chemosensory functions.
Limitation and bias in the study
The current study has one limitation, as the chemosensory function of patients was self-rated retrospectively before infection while smell, flavor, and taste were evaluated through the use of at-home testing. Thus, the patients may have retrospectively rated their chemosensory function too well. Moreover, the study may have a subject selection bias, as it included only patients with confirmed SARS-CoV-2 infection and subjective OD who were interested in the study.
OD treatment and future scope
As per the Position Paper on Olfactory Dysfunction, the recommended therapy for post-infectious OD is olfactory training. Several studies have reported the benefit of daily practicing using four odorants, which has now become the standard recommendation for olfactory training in COVID-19 patients with OD.
Different studies have also been conducted to evaluate the intranasal and oral administration of cortisone for OD in COVID-19 patients. While many of them did not reveal any significant improvement in COVID-19-related smell loss with cortisone, one case-control study showed significant improvement in psychophysically evaluated smell function with ten days of oral cortisone and after olfactory training.
Overall, the study concludes that even though earlier studies have reported that COVID-19 patients recover from OD within a few weeks, the findings discussed here show long-lasting OD in COVID-19 patients. The majority of patients had OD in the range of hyposmia, which was further confirmed by detailed smell tests.
Effective treatment methods to regain olfactory function are an important aspect of COVID-19 patient management. Further research is required regarding factors influencing prolonged COVID-19-related smell loss.